To satisfy the ever-increasing desire for faster and smaller electronic devices such as personal computers, it has become desirable to increase the number of microelectronic devices such transistors on a chip without increasing the size of the chip. Accordingly, it is desirable to continually strive to reduce the size of the microelectronic devices.
One of the primary hurdles in achieving the much sought after size reduction of microelectronic devices is in the area of photolithography. For decades, photolithography has been utilized to pattern photoresists in the manufacture of microelectronic devices. The resolution of the image formed on a photoresist layer using photolithography generally is directly proportional to the wavelength of the radiation source (λ) and inversely proportional to the numerical aperture (NA) of the photolithography apparatus. Thus, in order to reduce the feature size that can be patterned by a photolithography apparatus, it may be desirable to utilize radiation sources having shorter and shorter wavelengths and/or develop photolithography apparatus having larger numerical apertures.
Efforts have resulted in the reduction in wavelength from mercury g line (436 nm) to 193 nm using an excimer laser and further to 157 nm. Research is currently being performed to further reduce the wavelength of the radiation source using x-ray lithography and/or extreme ultraviolet (EUV) lithography. The cost of continuing to reduce the wavelength of the radiation source may be enormous. New materials for photomasks and/or lenses may need to be developed. As the wavelength becomes shorter, the photolithography method may need to shift from refractive photolithography to reflective photolithography. Designing an all-reflective camera that achieves lithographic-quality imaging may be more difficult than designing a refractive imaging system because mirrors generally have fewer degrees of freedom to vary than do lenses.
These challenges have resulted in an interesting intersection between microelectronic device manufacture and biology. When faced with the problem of increasing the resolution of microscope lenses beyond their normal magnification, biologists began placing a layer of oil between the lens and the slide to be examined. This technique, known as immersion oil microscopy, reduces the loss of image quality that would occur as a result of the difference in the refractive index between the glass of the lens and air. In an ideal situation, the refractive index of the oil is precisely matched to that of the glass so that the loss of image quality can be eliminated.
Using the principles of immersion oil microscopy, photolithographers have begun to explore an area that is coming to be known as immersion lithography. In immersion lithography, the space between the final optical element and the substrate to be patterned is at least partially filled with a high index medium. M. Switkes & M. Rothschild, “Immersion Lithography at 157 nm,” J. Vac. Sci. Technol. B, 19(6): 2353–2356 (November/December 2001) proposes the use of commercially available perfluoropolyethers (PFPE's), which are widely available as oils and lubricants, for example under the trade name Fomblin® (Solvay Solexis Corp.) as the high index medium in an immersion interference lithography system. Switkes & Rothschild utilized organic solvents such as Fomblin® PFS-1 to remove them from the patterned substrate. The Switkes & Rothschild publication is hereby incorporated herein by reference in its entirety as if set forth fully herein.
Immersion lithography has been regarded as a breakthrough technology that may allow the integration density of integrated circuit devices to continue to increase without the need for post-optical next generation lithography. See, for example, the publication entitled “‘Liquid Immersion’ could delay post-optical lithography, says MIT”, by Mark LaPedus, Semiconductor Business News, Mar. 11, 2002, and the publication entitled “What's Next: Full Immersion Lithography?” Solid State Technology, May 2002, Vol. 45, No. 5, p. 24.